Re: Right Semantics for ioremap, remap_page_range

[Richard B. Johnson]

On Wed, 18 Jul 2001, Alexander Ehlert wrote:

> Hi,
> 
> I'm currently trying to write a linux kernel driver for an experimental
> graphics board we're developing at our institute. It's fitted
> with an plx9054 and got some sdram on board connected to the plx.
> Now I come this far, that I actually detect the board, set some modes
> and do an ioremap on pci_resource_start(pdev,2) which is the
> base for 64Mb Ram Onboard. After ioremap() I actually like
> to do remap_page_range through fileops/mmap call. I just copied
> that code from drivers/char/mem.c, but just using the ioremapped
> address as offset in remap_page_range, doesn't seem to work, instead
> I think I just mmap some totally different area... Now, what do I have to
> use for that offset? What I currently do in the init function is
> something like that:
> 
> ...
> priv.pcibar2 = (char*)ioremap(pci_resource_start(pdev,2),
>                               pci_resource_len(pdev,2));
> ...

This is becoming a FAQ. The return value from ioremap() and friends
is not a pointer. It is actually something that from time-to-time
will be poisoned to detect its use as a pointer. It is a 'cookie'
designed to be used with readl() readw() readb(), writel(), etc.
For large arrays, you use copy/to/from_io().  It is possible to
determine the actual virtual address with a runtime code snippit
so you could access your remapped address conventionally, i.e.,
as a pointer, perhaps to a structure, etc., but cheating like
that is frowned upon and makes your driver more non-portable
than it probably already is. The assumption seems to be that
when Apple comes out with a 256 bit machine with 128 bit PCI
and 32, 40 GHz CPUs, you just recompile everything and it will run );

If your driver is never going to be used for anything but
a private experiment, the value of a pointer to the remapped
area is (usually) the (address_you_asked_for) | PAGE_OFFSET.

You have to save the returned cookie anyway because you use
it to release the remapped area when your module exits.

Cheers,
Dick Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

    I was going to compile a list of innovations that could be
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